Metal-insulator-metal (MIM) capacitors have been widely used in functional circuits such as mixed signal circuits, analog circuits, radio frequency (RF) circuits, dynamic random access memory (DRAM), embedded DRAM, and logic operation circuits. In system-on-chip applications, different capacitors for different functional circuits have to be integrated on a same chip to serve different purposes. For example, in mixed signal circuits, capacitors are used as decoupling capacitors and high-frequency noise filters. For DRAM and embedded DRAM circuits, capacitors are used for memory storage; while for RF circuits, capacitors are used in oscillators and phase-shift networks for coupling and/or bypassing purposes. For microprocessors, capacitors are used for decoupling. The traditional way to combine these capacitors on a same chip is to fabricate them in different metal layers.
With the capacitors having different functions formed in different metal layers, the capacitors may work under different operation voltages. For example, when used as decoupling capacitors, the capacitors need to be able to sustain high voltages. Therefore, the capacitor insulators need to be thick. In DRAMs, on the other hand, the operation voltage is low, and the capacitors need to be small in order to increase the DRAM cell density. Therefore, the capacitor insulators need to be thin.
The conventional capacitor integration scheme, however, suffers from drawbacks. With the capacitors for different functions formed in different layers, the capacitors in one metal layer need to have their own formation process that cannot be shared by other capacitors in different layers. For example, the bottom electrodes, the insulators, and the top electrodes in one metal layer have to be formed separately from the bottom electrodes, the insulators, and the top electrodes, respectively, of other capacitors in different layers. This significantly increases the cost and process complexity.